I work with some of the least computer savvy people on the planet. Even still, they want to be able to do things like work from home, so I set it up for them (I'm not any sort of official IT person, my job is completely unrelated, but I'm the closest they have and I cost a shitload less). I write out detailed instructions of how to log on step by step. For the most part it works and people are successful with little interaction from me.
Today, the person I would consider the most tech savvy, who has logged on remotely from home countless times, emails me that he can't on his new computer at home, and sends me a pic of his screen with the error (which says he's not authorized). The very first thing I ask him is if he's logging on following those steps I wrote out. He says yes. I say are you sure? Exactly like I spelled out? Yes, of course he is. So I start checking the server and his computer to make sure he has permission, because who knows, maybe it got reset somehow. I google solutions, check all sorts of shit, spend way more time than I wanted to on it, and come up blank. It should work. Because I think I know what it is, I tell him to make sure he is logging on with domain\username and not just username. He assures me that he is. So I make him send me a screenshot of the login screen. Of course, it shows computername\username, not the domain, meaning he only typed his username in like I told him not to. I tell him again to do it properly, just like I told him to do before, and like I had explicitly written out in the instructions. "That fixed it, thanks!"
